Oil and gas reservoirs often contain very large volumes of carbon dioxide and many of these are located in isolated offshore locations. Due to the location and environment of such offshore gas fields and the quantity of carbon dioxide, the cost of removing and processing the carbon dioxide is significantly higher than it would be otherwise. Some of these oil and gas reservoirs also contain other non-targeted component gases such as hydrogen sulphide, non-targeted produced water and non-targeted solids such as sands which need to be separated from the produced fluid stream and further processed or sequestered.
This cost is further increased when the greenhouse effects of releasing gases such as carbon dioxide into the atmosphere are considered. Once the greenhouse gas implications and costs are taken into account it becomes apparent that systems which enable the removal and processing or sequestration of the CO2 and other greenhouse gases such as H2S, whilst remaining compact enough for use in offshore locations, will be most favored.
Seawater or brine has previously been considered as a possible solvent/reactant for scrubbing power station flue gases in a conventional vertical tower scrubbing process located on land. The problems associated with this particular process are the large volumes of seawater required and the associated large vessels and significant pumping power required. Further, water at sea level and atmospheric pressure has a much lower capacity to absorb carbon dioxide than that it does at higher water pressure and lower water temperature.
Whilst the use of seawater for the removal of greenhouse gases would be advantageous in offshore installations, the large size of the apparatus for conventional vertical tower scrubbing processes makes their implementation highly impractical.
Further, the offshore location provides additional problems not faced in land installations as a result of greatly reduced system footprint availability and the increased operating pressures, especially in deep water, and the challenges faced with deployment and long term operation without regular vessel inspections.
Other attempts to use liquid solvents to remove greenhouse gasses from gas streams such as WO 2000074816 have utilised counter-current absorbers. However, these to experience a number of problems. Counter-current systems are heavily limited by gas velocity, due to the fact that above a certain gas velocity, counter-current systems will flood and entrain the liquid in the gas. Additionally, the solvent used (such as amines) will typically need to be regenerated, which requires the addition of further pressure vessels associated with regeneration. It is also the case that the absorbed gas is not easily sequestered.
Conventional counter-current absorbers are also limited by the fabrication limitations associated with large vessels operating under pressure. For large vessels this is around a design pressure of 100 bar. In order to accommodate this pressure requirement, the large pressure vessels have the following significant engineering and economic impacts:                Weight;        High fabrication costs (as special high cost alloys are required for certain applications such as carbon dioxide);        Large footprints, making them unsuitable for most offshore applications; and        Large Inventory (holding costs).        
There also remain safety concerns associated with the often dangerous, explosive and/or environmentally impacting inventory. In addition to the above problems, such pressure vessels cannot be safely deployed below depths of a few hundred meters.
Whilst co-current processes, such as those taught by EP 0180670, have been developed, these rely on atomised liquid droplets to perform the mass transfer function. Generally the efficiency of the mass transfer step is increased as the droplet size is decreased, as a higher liquid surface area contact is achieved with many small droplets as opposed to fewer larger ones. This type of process however does require a large vessel diameter to carry out the mass transfer process in order to prevent the droplets from coalescing together. According, these processes to suffer the aforementioned limitations.
Additionally the counter-current and co-current processes such as taught by WO 2000074816 and by EP 0180670 are not suitable for separating non-target liquids, such as water from the fluid streams produced at oil and gas reservoirs and non-target solids such as produced sands.
One object of the present invention is to provide a method for the removal of non-targeted components from targeted gas streams which can be utilised in the confined conditions of typical offshore or subsea locations. A further object is that the problems associated with increased operating pressures, deployments and the need for regular vessel inspections will be significantly reduced.
The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia or any other country or region as at the priority date of the application.
Throughout the specification, unless the context requires otherwise, the word “comprise” or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.
Throughout the specification, unless the context requires otherwise, the term “gas”, will be understood to be predominantly gas although it may contain some liquids and/or some solids.
Throughout the specification, unless the context requires otherwise, the term “liquid”, will be understood to be predominantly liquid although it may contain some gas or gases and/or some solids.
Throughout the specification, unless the context requires otherwise, the term “greenhouse gases”, will be understood to include any one or more of CO2, H2S, CO, HCl, CH4, N2O, CCl2F2, CHClF2, CF4, C2F6, SF6 and NF3.
Throughout the specification, unless the context requires otherwise, the term “inline process”, will be understood to include any process in which the elements of the process are pipe and comply with piping requirements. It will be understood that inline processes are not limited to those which are horizontal or in a straight line, but may include elements of any orientation including vertical elements.
Throughout the specification, unless the context requires otherwise, the terms “solvent” and “reactant”, will be understood to include any suitable fluid which can be any one or more of water, seawater, treated water, open source water, brine, and other aqueous and non-aqueous absorbents for gases including CO2 and other gases.
Throughout the specification, unless the context requires otherwise, the term “fluid”, will be understood to include any combination of gases, liquids and fluidised solids, including natural gas, condensate, oil and other hydrocarbons, water and sand.
Throughout the specification, unless the context requires otherwise, the term “non-targeted component”, will be understood that or those components not desired to be present above certain levels in the target fluid stream produced after passing through the invention.
Throughout the specification, the term “non-targeted” should not be construed to mean of no commercial value. The non-targeted components may have significant commercial value. The terms “non-targeted” and “targeted” are used to distinguish one component or combination of components from another component or combination of components.
Throughout the specification, unless the context requires otherwise, the term “purified gas”, will be understood to mean gas with a reduced level of non-targeted components.
Throughout the specification, unless the context requires otherwise, the term “wet gas”, will be understood to mean gas after the solvent has been added into it.
Throughout the specification, unless the context requires otherwise, the term “dry gas”, will be understood to mean gas after the bulk of the solvent has been separated from it.
Throughout the specification, unless the context requires otherwise, the term “rich”, will be understood to mean containing a high level of non-targeted component.
Throughout the specification, unless the context requires otherwise, the term “lean”, will be understood to mean containing a low level of non-targeted component.